-1, Can you be more specific in what you're looking for? Are you looking for an actual simultaneous user editing environment for textures? A network supported multi-user code IDE? Or just something more mundane like some cloud storage so people can work on "the same file" at different times?
–
Tetrad♦Nov 1 '10 at 20:41

Try GitHub for version control. It's fantastic -- you can use git or svn (whichever you're more comfortable with) and your repositories get free issue tracking, wikis and cool visual graphs. The only downside is that if you want your project hosted privately, you have to pay. (It's as low as $7/month as of this writing; we pay and find it well worth the cost.)

File Sharing:

We also love and use Dropbox. It supports Windows, Mac and Linux and features automatic file syncing across multiple machines. You can even hook it into Growl to see what your collaborators are doing realtime! Best of all, the basic plan is free and gives you a very generous 2GB of storage. (Note: should be mentioned to be careful with Dropbox files, as your collaborators could inadvertently destroy your changes. Luckily Dropbox provides some versioning but extra precautions on your end are recommended!)

+1 for mentioning Dropbox, this program has been incredibly awesome to me as well.
–
Bryan HarringtonNov 2 '10 at 2:08

Careful using Dropbox for multi-user things though. If two people edit something at the same time, you end up with two copies of it, one renamed. It can cause more trouble than it's worth, to have to open and compare the two copies and hand-merge them. Not that it's Dropbox's fault at all (this is the best thing it could possibly do) but obviously it can't make assumptions and merge things for you like a version control system does. You'd be better off even with something old like CVS (check in, check out) or else you'll replicate it by sending emails: "I'm working on this, don't touch it!"
–
RicketNov 2 '10 at 3:13